New Topology for voltage balancing in railway substations G. Raimondo*, P. Ladoux*, P. Marino**, H. Caron*** * Université de Toulouse INPT, UPS, CNRS LAPLACE (Laboratoire Plasma et Conversion d'Energie) ENSEEIHT 2, rue Charles Camichel BP 7122 F-31071 Toulouse Cedex 7, France (email: giuliano.raimondo@laplace.univ-tlse.fr, philippe.ladoux@laplace.univ-tlse.fr) ** Seconda Università degli Studi di Napoli , SUN, Dipartimento di Ingegneria dell’Informazione (DII) Via Roma 29, 81031 Aversa (CE), Italy (email: pompeo.marino@unina2.it) *** Société Nationale des Chemins de fer Français (SNCF), Direction de l'Ingénierie 6, avenue François Mitterrand, 93574 La Plaine Saint-Denis, France (email: herve.caron@sncf.fr ) Abstract: This paper deals with a new technique of voltage unbalance compensation for single phase railway substations. Using the concept of Chopper Controlled Impedance, the authors study the feasibility of an active Steinmetz circuit based on AC choppers. The low power losses in semiconductor devices make the proposed topology an interesting alternative to classical solution like VSI based. Design criteria are presented using information got from measurements carried out in a substation of the French Railways. Finally, the study is validated by means of simulations and experimental results carried out on a 1.5 kVA prototype. Keywords: Unbalance compensation, AC Chopper, Steinmetz Circuit, Railway substations. 1. INTRODUCTION Nowadays, high speed railways are powered with a 25 kV/50 Hz single phase AC line. The substations are phase to phase connected to transmission lines. They behave as nonlinear and time-varying loads and represent one of the most important sources of voltage unbalance regarding the electricity transmission network. In case of weak networks, railways operators are required to install compensation systems in substations in order to satisfy limits and to avoid penalty. The limits are established by the energy provider with a view to guarantee a sufficient voltage quality to other customers. For ten years, the Voltage Source Inverter topology is widely used as unbalance compensation system. Nevertheless, in high power applications, the losses in semiconductor devices show significant costs both in terms of active energy and cooling system maintenance. In the case of railway power systems this criterion has to be taken into consideration by the operator. The concept of Chopper Controlled Impedance (CCI) was presented by Ladoux et al. (2008, 2010) as a low power losses solution for reactive power compensation in railways substations. This topology is based on series or parallel associations of AC-Choppers to carry out controllable impedances at the network frequency. A common solution to rebalance industrial high-power single-phase loads is the Steinmetz circuit (Sainz et al. 2005). This technique consists of an inductor and a capacitor suitably connected to the three-phase network to rebalance the single-phase load. In this paper, the authors study the feasibility of an Active Steinmetz unbalance compensator based on the CCI concept. Firstly the proposed compensator topology is introduced and advantages and drawbacks in terms of compensation capability and semiconductor power losses are highlighted. A substation of the French Railways is considered as a case study. A measurement campaign allows obtaining useful information regarding the unbalance compensator rating. The design methodology for the Active Steinmetz compensator is presented. Finally, simulation results and experimental results are presented. 2. VOLTAGE UNBALANCE COMPENSATION 3.1 Substation Unbalance Figure 1 shows the scheme of a classical railway substation supplied by a three-phase network. At the Point of Common Coupling (PCC), in order to avoid penalties from the energy provider, the railways company is forced to meet a maximum voltage Unbalance Factor (UF) on 10 minutes average. Fig. 1. Single-Phase Substation connection UF is defined as the ratio of the negative sequence component V - and the positive sequence component V + of the line voltages (v A ,v B ,v C ). A good approximation for the unbalance factor is: % 100 100 cos( ) cc cc cc I Z I V UF E Z I V ϕ ϕ + - - + + + = ⋅ ≈ ⋅ - - (1) Where: A e cc Z B e C e A i B i C i L i PCC A v B v C v cc Z cc Z train i